FUEL CELLS Problems and Solutions
VLADIMIR S. BAGOTSKY A.N. Frumkin Institute of Electrochemistry and Physical Chemistry Russian Academy of Sciences Moscow, Russia
FUEL CELLS
FUEL CELLS Problems and Solutions
VLADIMIR S. BAGOTSKY A.N. Frumkin Institute of Electrochemistry and Physical Chemistry Russian Academy of Sciences Moscow, Russia Copyright r 2009 by John Wiley & Sons, Inc. All rights reserved.
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Library of Congress Cataloging-in-Publication Data:
Bagotsky, V. S. (Vladimir Sergeevich) Fuel cells: problems and solutions/Vladimir Bagotsky p. cm. Includes index. ISBN 978-0-470-23289-7 (cloth) 1. Fuel cells. I. Title.
TK2931.B35 2008 621.31u2429–dc22 2008033276
Printed in the United States of America
10987654321 CONTENTS
PREFACE xi SYMBOLS xiii ACRONYMS AND ABBREVIATIONS xv
I INTRODUCTION 1
INTRODUCTION 3
1 THE WORKING PRINCIPLES OF A FUEL CELL 7 1.1 Thermodynamic Aspects 7 1.2 Schematic Layout of Fuel Cell Units 11 1.3 Types of Fuel Cells 15 1.4 Layout of a Real Fuel Cell: The Hydrogen–Oxygen Fuel Cell with Liquid Electrolyte 15 1.5 Basic Parameters of Fuel Cells 20 Reference 26
2 THE LONG HISTORY OF FUEL CELLS 27 2.1 The Period Prior to 1894 27 2.2 The Period from 1894 to 1960 30 v vi CONTENTS
2.3 The Period from 1960 to the 1990s 33 2.4 The Period After the 1990s 39 References 40
II MAJOR TYPES OF FUEL CELLS 43
3 PROTON-EXCHANGE MEMBRANE FUEL CELLS 45 3.1 History of the PEMFC 46 3.2 Standard PEMFC Version from the 1990s 49 3.3 Special Features of PEMFC Operation 54 3.4 Platinum Catalyst Poisoning by Traces of CO in the Hydrogen 57 3.5 Commercial Activities in Relation to PEMFCs 59 3.6 Future Development of PEMFCs 60 3.7 Elevated-Temperature PEMFCs 67 References 70
4 DIRECT LIQUID FUEL CELLS 73
PART A: DIRECT METHANOL FUEL CELLS 73 4.1 Methanol as a Fuel for Fuel Cells 73 4.2 Current-Producing Reactions and Thermodynamic Parameters 74 4.3 Anodic Oxidation of Methanol 74 4.4 Milestones in DMFC Development 76 4.5 Membrane Penetration by Methanol (Methanol Crossover) 77 4.6 Varieties of DMFCs 79 4.7 Special Operating Features of DMFCs 81 4.8 Practical Models of DMFCs and Their Features 83 4.9 Problems To Be Solved in Future DMFCs 85
PART B: DIRECT LIQUID FUEL CELLS 87 4.10 The Problem of Replacing Methanol 87 4.11 Fuel Cells Using Organic Liquids as Fuels 88 4.12 Fuel Cells Using Inorganic Liquids as Fuels 94 References 97 CONTENTS vii
5 PHOSPHORIC ACID FUEL CELLS 101 5.1 Early Work on Phosphoric Acid Fuel Cells 101 5.2 Special Features of Aqueous Phosphoric Acid Solutions 102 5.3 Construction of PAFCs 103 5.4 Commercial Production of PAFCs 104 5.5 Development of Large Stationary Power Plants 105 5.6 The Future of PAFCs 105 5.7 Importance of PAFCs for Fuel Cell Development 107 References 107
6 ALKALINE FUEL CELLS 109 6.1 Hydrogen–Oxygen AFCs 110 6.2 Alkaline Hydrazine Fuel Cells 117 6.3 Anion-Exchange (Hydroxyl Ion–Conducting) Membranes 121 6.4 Methanol Fuel Cells with Anion-Exchange Membranes 122 6.5 Methanol Fuel Cell with an Invariant Alkaline Electrolyte 123 References 123
7 MOLTEN CARBONATE FUEL CELLS 125 7.1 Special Features of High-Temperature Fuel Cells 125 7.2 Structure of Hydrogen–Oxygen MCFCs 126 7.3 MCFCs with Internal Fuel Reforming 128 7.4 Development of MCFC Work 130 7.5 The Lifetime of MCFCs 131 References 133
8 SOLID-OXIDE FUEL CELLS 135 8.1 Schematic Design of Conventional SOFCs 136 8.2 Tubular SOFCs 138 8.3 Planar SOFCs 143 8.4 Monolithic SOFCs 146 8.5 Varieties of SOFCs 147 8.6 Utilization of Natural Fuels in SOFCs 149 8.7 Interim-Temperature SOFCs 151 8.8 Low-Temperature SOFCs 155 8.9 Factors Influencing the Lifetime of SOFCs 157 References 158 viii CONTENTS
9 OTHER TYPES OF FUEL CELLS 161 9.1 Redox Flow Cells 161 9.2 Biological Fuel Cells 164 9.3 Semi-Fuel Cells 167 9.4 Direct Carbon Fuel Cells 170 References 174
10 FUEL CELLS AND ELECTROLYSIS PROCESSES 177 10.1 Water Electrolysis 177 10.2 Chlor-Alkali Electrolysis 182 10.3 Electrochemical Synthesis Reactions 185 References 187
III INHERENT SCIENTIFIC AND ENGINEERING PROBLEMS 189
11 FUEL MANAGEMENT 191 11.1 Reforming of Natural Fuels 192 11.2 Production of Hydrogen for Autonomous Power Plants 196 11.3 Purification of Technical Hydrogen 199 11.4 Hydrogen Transport and Storage 202 References 205
12 ELECTROCATALYSIS 207 12.1 Fundamentals of Electrocatalysis 207 12.2 Putting Platinum Catalysts on the Electrodes 211 12.3 Supports for Platinum Catalysts 214 12.4 Platinum Alloys and Composites as Catalysts for Anodes 217 12.5 Nonplatinum Catalysts for Fuel Cell Anodes 219 12.6 Electrocatalysis of the Oxygen Reduction Reaction 221 12.7 The Stability of Electrocatalysts 227 References 228
13 MEMBRANES 231 13.1 Fuel Cell–Related Membrane Problems 232 13.2 Work to Overcome Degradation of Nafion Membranes 233 CONTENTS ix
13.3 Modification of Nafion Membranes 233 13.4 Membranes Made from Polymers Without Fluorine 235 13.5 Membranes Made from Other Materials 237 13.6 Matrix-Type Membranes 237 13.7 Membranes with Hydroxyl Ion Conduction 238 References 239
14 SMALL FUEL CELLS FOR PORTABLE DEVICES 241 14.1 Special Operating Features of Mini-Fuel Cells 242 14.2 Flat Miniature Fuel Batteries 243 14.3 Silicon-Based Mini-Fuel Cells 245 14.4 PCB-Based Mini-Fuel Cells 247 14.5 Mini-Solid Oxide Fuel Cells 248 14.6 The Problem of Air-Breathing Cathodes 249 14.7 Prototypes of Power Units with Mini-Fuel Cells 250 14.8 Concluding Remarks 253 References 253
15 MATHEMATICAL MODELING OF FUEL CELLS 255 Felix N. Bu¨chi
15.1 Zero-Dimensional Models 257 15.2 One-Dimensional Models 257 15.3 Two-Dimensional Models 258 15.4 Three-Dimensional Models 259 15.5 Concluding Remarks 260 References 260
IV COMMERCIALIZATION OF FUEL CELLS 263
16 APPLICATIONS 265 16.1 Large Stationary Power Plants 265 16.2 Small Stationary Power Units 269 16.3 Fuel Cells for Transport Applications 272 16.4 Portables 277 16.5 Military Applications 281 References 283 x CONTENTS
17 FUEL CELL WORK IN VARIOUS COUNTRIES 285 17.1 Driving Forces for Fuel Cell Work 285 17.2 Fuel Cells and the Hydrogen Economy 287 17.3 Activities in North America 289 17.4 Activities in Europe 290 17.5 Activities in Other Countries 291 17.6 The Volume of Published Fuel Cell Work 294 17.7 Legislation and Standardization in the Field of Fuel Cells 295 References 296
18 OUTLOOK 297 18.1 Periods of Alternating Hope and Disappointment 297 18.2 Some Misconceptions 299 Klaus Mu¨ller 18.3 Ideal Fuel Cells 300 18.4 Projected Future of Fuel Cells 302 References 304
GENERAL BIBLIOGRAPHY 305
AUTHOR INDEX 309
SUBJECT INDEX 315 PREFACE
When fuel cells were first suggested and discussed, in the nineteenth century, it was firmly hoped that distinctly higher efficiencies could be attained with them when converting the chemical energy of natural fuels to electric power. Now that the world supply of fossil fuels is seen to be finite, this hope turns into a need: into a question of maintaining advanced standards of living. Apart from conversion efficiency, fuel cells have other aspects that make them attractive: Their conversion process is clean, they may cogenerate useful heat, and they can be used in a variety of fields of application. One worker in the field put it this way: ‘‘Fuel cells have the potential to supply the electricity powering a wristwatch or a large city, replacing a tiny battery or an entire power generating station.’’ With some important achievements made in the past, fuel cells today are a subject of vigorous R&D, engineering, and testing conducted on a broad international scale in universities, research centers, and private companies in various sectors of the economy. Combining engineers, technicians, and scien- tists, several 10,000 workers contribute their efforts and skills to advancing the field. Progress in the field is rapid. Each month hundreds of publications report new results and discoveries. Important synergies exist with work done to advance the concepts of a hydrogen economy. The book is intended for people who have heard about fuel cells but ignore the detailed potential and applications of fuel cells to focus on the information they need: engineers in civil, industrial, and military jobs; R&D people of diverse profile; investors; decision makers in government, industry, trade, and all levels of administration; journalists; school and university teachers and
xi xii PREFACE students; and hobby scientists. The work is also intended for people in industry and research who in their professional work are concerned with various special aspects of the development and applications of fuel cells and want to gain an overview of fuel cell problems and their economic and scientific significance. The aim of this book is to provide readers across trades and lifestyles with a compact, readable introduction and explanation of what fuel cells do, how they do it, where they are important, what the problems are, and how they will continue in the field: what they could do against air pollution and for portable devices. All this is done with a critical attitude based on a detailed and advanced presentation. Problems and achievements are discussed at the level attained by the end of 2007. Contradictions and a lack of consensus have existed in the field, along with ups and downs. In a field where the subject may range in size from milliwatt to megawatt output, and where many technical systems compete, this will not come as a surprise. To guide the reader through the maze, a sampling of literature references is provided. Unfortunately, a lot of work just as important as the work cited had to be omittted. Selection was also made difficult because of the strongly interdisciplinary character of fuel cell work. The presentation is made against the historical background, and looks at future prospects, including those of a synergy with a potential future hydrogen economy. Where views diverge, they are presented as such. Some of the ideas offered may well be open to further discussion. My sincere thanks are due Dr. Felix Bu¨ chi of the Paul Scherrer Institute in Villigen, Switzerland, who contributed the important chapter on the modeling of fuel cells. My gratitude goes to my colleagues the late Dr. Nina Osetrova and to Dr. Alexander Skundin, of Moscow, for their help in selecting relevant literature, and to Timophei Pastushkin for preparing graphical representations. My thanks also go to Dr. Klaus Mu¨ ller, formerly at the Battelle Institute of Geneva, who transformed chapters written in Russian into English, contrib- uted Section 18.2, and made a number of very valuable suggestions. I sincerely hope that what has inspired me during more than 50 years of research and teaching at the Moscow Quant Power Sources Institute and the A.N. Frumkin Institute of Electrochemistry and Physical Chemistry, Russian Academy of Sciences, will continue to inspire current and future specialists and people in general who work to improve our lives and solve our problems.
VLADIMIR SERGEEVICH BAGOTSKY
Moscow, Russia and Mountain View, California May 2008 E-mail:[email protected] SYMBOLS
Dimensions Symbol Meaning (values) Section*
ROMAN SYMBOLS
3 cj concentration mol/dm 2 Dj diffusion coefficient cm /s E electrode potential V 1.4.3 E0 equilibrium electrode potential V 1.4.3 F Faraday constant 94850 C/mol 7.2 G Gibbs energy kJ/mol 1.1.2 H enthalpy kJ/mol 1.1.2 i current density mA/cm2 1.4.3 i0 exchange current density mA/cm2 1.4.3 I current A, mA 1.4.3 M mass kg molar concentration mol/dm3 n number of electrons in the reaction’s none 1.4.2 elementary act p power density W/kg 1.5.5 power W, kW 1.5.2 q heat (in eV) eV 1.4.2 Q heat, thermal energy J, kJ 1.1. 1
*Section where this symbol is used for the first time and/or where it is defined.
xiii xiv ACRONYMS AND ABBREVIATIONS
R resistance O 1.4.3 molar gas constant 8.314 J/mol K 7.2 S entropy kJ/K 1.1.2 surface area cm2 T absolute temperature K 1.1.1 U cell voltage V 1.4.4 w energy density kWh/kg 1.5.5 W work, useful energy W, kW 1.1.2
GREEK SYMBOLS g roughness factor none d thickness cm E0 electromotive force V 1.4.4 le amount of coulombs none 1.5.3 Z efficiency none, % s conductivity S/cm2
SUBSCRIPTS ads adsorbed app apparent e electrical exh exhaust ext external h.e. hydrogen electrode i under current j any ion, substance loss energy loss o.e. oxygen electrode ox oxidizer S per unit area red reducer V per unit volume 0 without current + cation anion ACRONYMS AND ABBREVIATIONS*
ac alternating current AFC alkaline fuel cell APU auxiliary power unit ATR autothermal reforming CD current density CHP combined heat and power CNT carbon nanotube CTE coefficient of thermal expansion DBHFC duirect borohydride fuel cell dc direct current DCFC direct carbon fuel cell DEFC direct ethanol fuel cell DFAFC direct formic acid fuel cell DHFC direct hydrazine fuel cell DLFC direct liquid fuel cell DMFC direct methanol fuel cell DSA dimensionally stable anode EMF electromotive force EPS electrochemical power source ET-PEMFC elevated-temperature PEMFC FCI Fuel Cells International FCV fuel cell vehicle GDL gas-diffusion layer GLDL gas–liquid diffusion layer ICV internal combustion vehicle IRFC internal reforming fuel cell IT-SOFC interim-temperature SOFC LHV lower heat value LT-SOFC low-temperature SOFC MCFC molten arbonate fuel cell MEA membrane–electrode assembly OCP open-circuit potential OCV open-circuit voltage ORR oxygen reduction reaction Ox, ox oxidized form PAFC phosphoric acid fuel cell
* These acronyms and abbreviations are used in most chapters. Acronyms for oxide materials used as electrolytes and electrodes in solid-oxide fuel cells are given in Chapter 8. xvi ACRONYMS AND ABBREVIATIONS
PBI polybenzimidazole PCB printed circuit board PD potential difference PEEK polyether ether ketone PEMFC proton-exchange membrane fuel cell (polymer electrolyte membrane fuel cell) PFSA perfluorinated sulfonic acid POX partial oxidation (reforming by) PVD physical vapor deposition Red, red reduced form SHE standard hydrogen electrode SOFC solid-oxide fuel cell SR steam reforming URFC unitized regenerative fuel cell UCC Union Carbide Corporation UTC United Technologies Corporation WGSR water-gas shift reaction PART I
INTRODUCTION
1
INTRODUCTION
Fuel cells have the potential to supply electricity to power a wrist watch or a large city, replacing a tiny battery or a power generating station. — George Wand, Fuel cell history, Part 1, Fuel Cells Today, April 2006
What Is a Fuel Cell? Definition of the Term A fuel cell may be one of a variety of electrochemical power sources (EPSs), but is more precisely a device designed to convert the energy of a chemical reaction directly to electrical energy. Fuel cells differ from other EPSs: the primary galvanic cells called batteries and the secondary galvanic cells called accumu- lators or storage batteries, (1) in that they use a supply of gaseous or liquid reactants for the reactions rather than the solid reactants (metals and metal oxides) built into the units; (2) in that a continuous supply of the reactants and continuous elimination of the reaction products are provided, so that a fuel cell may be operated for a rather extended time without periodic replacement or recharging. Possible reactants or fuels for the current-producing reaction are natural types of fuel (e.g., natural gas, petroleum products) or products derived by fuel processing, such as hydrogen produced by the reforming of hydrocarbon fuels
Fuel Cells: Problems and Solutions, By Vladimir S. Bagotsky Copyright r 2009 John Wiley & Sons, Inc.
3 4 INTRODUCTION or water gas (syngas) produced by treating coal with steam. This gave rise to their name: fuel cells.
Significance of Fuel Cells for the Economy In this book we show that fuel cells, already used widely throughout the economy, offer: